a) Field of the Invention
The present invention is directed to an arrangement for switching high electric currents by way of a gas discharge at high voltages or for generating gas discharge plasma emitting EUV radiation, comprising an anode and a cathode which are both shaped so as to be hollow in a rotationally-symmetric manner and through which a discharge space is formed in the interior of the anode, wherein the cathode has a hollow cathode space for pre-ionization of a work gas and the hollow cathode space is delimited relative to the discharge space by a metal wall with a plurality of openings for streaming pre-ionized work gas into the discharge space, these openings being arranged at regular spatial intervals. It is applied particularly in gas discharge arrangements for generating plasma that emits EUV radiation in radiation sources for semiconductor lithography and pseudospark switches.
b) Description of the Related Art
Special gas discharge arrangements for generating short-wavelength radiation are operated by electrically pulsed high-power sources. In the simplest case, they are capacitors which are charged by line voltage equipment and then discharged when an electric contact is closed by suitable switches by means of a gas discharge arrangement. Peak currents of up to 50 kA at voltages of more than 5 kV with rates of current rise greater than 1 kA/ns must be handled. Pseudospark switches which are described, e.g., in U.S. Pat. No. 6,417,604 B1, U.S. Pat. No. 5,502,356 A, U.S. Pat. No. 5,126,638 A and U.S. Pat. No. 5,399,941 A are suitable for this purpose.
Pseudospark switches are gas-filled discharge arrangements with electrodes comprising one or more discharge openings arranged in a suitable geometric manner. These openings cause directed, stable discharges. The purpose of using a plurality of discharge channels is to reduce the local current density. The gas pressure and electrode spacing are selected in such a way that the operating point lies on the left-hand side of the Paschen curve. The cathode is preferably shaped as a hollow cathode, and one or more trigger openings in an intermediate wall of the cathode make it possible to ignite a hollow cathode plasma.
By abstracting from the physical functional principle, it can be seen that the essential difference between pseudospark switches and gas discharge radiation sources merely consists in that the latter has an additional anode opening for radiation emission. Therefore, the functionality (useful life) can be prolonged in both cases of application by improving the design of the hollow cathode.
The conventional arrangements of gas discharge radiation sources and pseudospark switches have two substantial disadvantages which severely limit the life of the current-loaded electrodes:    a) The geometry of known pseudospark switches and radiation sources based on hollow cathode gas discharges does not permit a high-power cooling of the cathode. High-power operation of such switches (repetition frequencies of greater than 4 kHz) requires the dissipation of an average heat output of several tens of kW.    b) The thickness of the metal wall which separates the discharge space from the hollow cathode space is usually about 1 to 3 mm. This severely limits the life of the cathode, which is exacerbated by the poor dissipation of heat.
It is easily recognized that the cause of the short life of the cathode is the functionally important metal wall between the hollow cathode space and the main discharge space because, on one hand, it is the quickest to become worn in such a way as to impair function due to the ion erosion and, on the other hand, is simply too thin for a high-power cooling for reducing erosion. However, increasing the wall thickness, which would obviously substantially prolong the useful life of the wall against erosion, would bring about a change in the discharge behavior due to the substantial lengthening of the discharge holes in the cathode wall.
In contrast to the conventional hollow cathode structure in which the cathode wall—as described, e.g., in US2006/0138960 A1—has a plurality of uniformly arranged openings in a sieve-like manner, the attainable current strengths are diminished when the wall thickness is increased because of the relatively long through-openings so that the hollow cathode plasma no longer leads to the desired stable gas discharge in the discharge space.